Gas turbine engines may be used to power various types of vehicles and systems, such as air or land-based vehicles. In typical gas turbine engines, compressed air generated by axial and/or radial compressors is mixed with fuel and burned, and the expanding hot combustion gases are directed along a flowpath and through a turbine nozzle having stationary vanes. The combustion gas flow deflects off of the vanes and impinges upon turbine blades of a turbine rotor. A rotatable turbine disk or wheel, from which the turbine blades extend, spins at high speeds to produce power. Gas turbine engines used in aircraft use the power to draw more air into the engine and to pass high velocity combustion gas out of the gas turbine aft end to produce a forward thrust. Other gas turbine engines may use the power to turn a propeller or an electrical generator.
Typically, the stationary vanes of the turbine nozzle extend between an inner endwall ring (also known as a “hub ring”) and an outer endwall ring (also known as a “shroud ring”). The inner and outer endwall rings define a portion of the flowpath along which the combustion gas travels. In some cases, the inner and/or outer endwalls rings are initially formed as segments, and the segments are subsequently assembled together to form a full ring (a “conventional segmented turbine nozzle”). Conventional segmented turbine nozzles may experience significant leakage where the adjacent segments meet at segment platform seal gaps and intermittent flange surfaces. Additionally, high leakage may exist where the segments mate to the supporting structure due to dimensional variation caused by individually machined segments. The leakage between segments is detrimental to the gas turbine engine in two major ways. First, the leakage increases chargeable cooling flow that does not get turned by the turbine nozzle to produce work across the turbine rotor, thus increasing fuel consumption. Secondly, the increased leakage flow wastes precious cooling flow that could be used for combustor and turbine component cooling. As combustor and turbine nozzle distress are among the top contributors to hot section replacement overhaul costs, gas turbine engine designers are eagerly seeking ways to reduce this detrimental leakage in segmented turbine nozzles and use the flow to cool the combustor and nozzle instead, thereby improving component durability and service life.
Turbine nozzles may also be manufactured by bi-casting the stationary turbine vanes with the inner and outer endwall rings, so that the rings and the vanes comprise a single, unitary turbine nozzle (a “conventional bi-cast turbine nozzle”). Though bi-cast inner and outer endwall rings reduce turbine nozzle leakage, they may be difficult and/or time consuming to manufacture, with reduced manufacturing yields. For example, a bi-cast turbine nozzle may suffer cracking distress and reduced service life due to thermo-mechanical fatigue (TMF) caused by a lack of radial compliance between the vanes and endwall rings. In addition, bi-casting of the endwall rings requires that the endwall rings be fabricated from an equi-axed alloy that has lower strength and oxidation capabilities than a single crystal alloy. Moreover, protective coatings may be relatively difficult to apply to conventional bi-cast turbine nozzles. The coated surfaces of conventional bi-cast turbine nozzles show irregularities on the surfaces where “shadows” cast by adjacent vanes result in a non-optimal coating microstructure and thickness distribution.
Hence, there is a need for improved methods for manufacturing a turbine nozzle with single crystal alloy nozzle segments. There is also a need for improved methods for manufacturing a turbine nozzle with single crystal alloy nozzle segments to reduce leakage and improve coating application. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.